1. Field of the Invention
The present invention provides a novel polarization-maintaining optical fiber coupler which is useful in the optical fiber communication field, the field of sensors using optical fibers and the like, and which couples and branches lights while maintaining the polarization of light in optical fibers. This application claims the priority of Japanese Patent Application No. 11-153080, which is incorporated herein by reference.
2. Description of the Related Art
The mode of light is comprised of an X polarized wave and Y polarized wave. A device which can couple and branch those polarized waves is called a polarization beam splitter (hereinafter abbreviated to xe2x80x9cPBSxe2x80x9d). A PBS is useful, for example, in a fiber optic gyro which measures the angular velocity using, for example, the interference of light or in coupling and branching lights from a light source which has linear polarization. To realize the characteristics of a PBS, the X polarized wave and Y polarized wave should have different coupling characteristics.
Proposed as such an optical device is a polarization-maintaining optical fiber coupler which uses polarization-maintaining optical fibers.
Various kinds of polarization-maintaining optical fibers have been proposed so far, and a typical known one is a PANDA (Polarization maintaining AND Absorption reducing) fiber.
FIG. 12 exemplifies a PANDA fiber. This PANDA fiber 10 comprises a core 11 provided at the center, a cladding 12 provided concentrical to the core 11 and having a lower refractive index than that of the core 11, and two stress applying sections 13 arranged in the cladding 12 symmetrically to each other around the core 11 and having a circular cross section and a lower refractive index than the cladding 12.
In this example, the core 11 is formed of germanium-doped quartz glass, the cladding 12 is formed of pure quartz glass, and each stress applying section 13 is formed of quartz glass in which a relatively large amount of boron is doped. The outside diameter of the core 11, the outside diameter of the stress applying section 13, the relative refractive-index difference between the core 11 and the cladding 12, and the relative refractive-index difference between the cladding 12 and the stress applying section 13 are adequately set in accordance with the desired characteristics. The outside diameter of the cladding 12 is normally set to approximately 125 xcexcm.
The stress applying section 13 has a larger coefficient of thermal expansion than the cladding 12. In the process where the optical fiber drawn at the time of production is cooled, strain originated at the stress applying section 13 is applied to the fiber""s cross section.
This strain produces anisotropic strain with respect to the core 11, clearing the degeneracy of polarized waves so that the propagation constant of the X polarized wave differs from that of the Y polarized wave. Naturally, the distributions of the electromagnetic fields of those polarized waves differ from each other. This provides the characteristic such that the X polarized wave and Y polarized wave are maintained while propagating.
FIG. 13 exemplifies a polarization-maintaining optical fiber coupler. This polarization-maintaining optical fiber coupler 14 has two PANDA fibers 10 arranged side by side in such a way that their axes of polarization become parallel to each other. The PANDA fibers 10 are heated and melted with claddings 12 midways of the PANDA fibers 10 and are elongated in the lengthwise direction, thus forming a fused-elongated section (optical coupling section) 3. Note that the axis of polarization is the line in each PANDA fiber 10 that passes the center between the stress applying sections 13.
In this polarization-maintaining optical fiber coupler, the X polarized wave propagates while maintaining the electric field vector in the direction of the polarization axes of the PANDA fibers 10, while the Y polarized wave propagates in the PANDA fibers 10 while maintaining the electric field vector in the direction perpendicular to the direction of the former electric field vector. The X polarized wave and Y polarized wave are coupled or branched at the fused-elongated section 3 at a midway.
According to the conventional polarization-maintaining optical fiber coupler, the difference between the coupling ratio of the X polarized wave and that of the Y polarized wave can be provided by making long the elongation length, namely the length by which the optical fiber (PANDA fiber 10) is to be elongated at the time the fused-elongated section 3 is formed. This difference can provide the conventional polarization-maintaining optical fiber coupler with the characteristics of a PBS.
FIG. 14A is a graph showing the relationship between the elongation length and the coupling ratio of light having a wavelength in use. The broken line represents the coupling characteristic of the X polarized wave, and the solid line the coupling characteristic of the Y polarized wave.
Forming the fused-elongated section of the conventional polarization-maintaining optical fiber coupler involves the repetition of an operation of coupling both the X polarized wave and Y polarized wave from one polarization-maintaining optical fiber (first optical fiber) to the other polarization-maintaining optical fiber (second optical fiber), further proceeding elongation to thereby transfer (couple) both polarized waves to the first optical fiber, and then transferring the polarized waves to the second optical fiber.
In forming the fused-elongated section 3 using ordinary polarization-maintaining optical fibers, the coupling of the Y polarized wave is slightly larger than the coupling of the X polarized wave, thus providing a slight difference between the cyclic changes (transfer cyclic changes) in the coupling ratios of the Y polarized wave and the X polarized wave. For the sake of convenience, one cycle is taken as a change in the coupling ratio which starts increasing from 0%, reaches 100%, then decreases to 0%, and two cycles are simply the repetition of one cycle twice.
When the elongation length becomes longer and the number of cycles becomes several cycles to several tens of cycles, the difference between the coupling ratios of the X polarized wave and the Y polarized wave becomes larger. If the fused-elongated section 3 is formed, elongated to the vicinity of the point where the difference in the coupling ratio indicated by the thick arrow in the graph becomes large, it is possible to acquire the characteristics of a PBS such that when the X polarized wave and Y polarized wave of the wavelength in use are input from the input-side port which is comprised of the same fiber as an output-side port A, the X polarized wave is output from the output-side port A and the Y polarized wave is output from the other port B.
The conventional polarization-maintaining optical fiber coupler however suffers the problem of the long device length needed to couple and branch the X polarized wave and the Y polarized wave. With the use of a polarization-maintaining optical fiber having an outside diameter of 125 xcexcm, for example, the elongation length would become more than 60 mm and would become as long as about 100 mm in some cases.
This long length makes the fused-elongated section very thin and inevitably reduces the mechanical strength and requires reinforcement. However, reinforcement is difficult to achieve because attaching a reinforcing member to the fused-elongated section alters the optical characteristics.
In addition, the wavelength band that permits coupling and branching of the X polarized wave and Y polarized wave is extremely narrow, for example, as narrow as about 10 nm.
Accordingly, it is an object of the present invention to provide a polarization-maintaining optical fiber coupler which has a shorter fused-elongated section than the conventional one and whose coupling ratio has a large dependency on polarization.
It is another object of this invention to provide a polarization-maintaining optical fiber coupler which can improve the mechanical strength.
It is a further object of this invention to provide a polarization-maintaining optical fiber coupler having a polarization dependency which can be used over a wide wavelength band.
To achieve the above objects, according to one aspect of this invention, there is provided a method of manufacturing a polarization-maintaining optical fiber coupler, which comprises the steps of heating lengthwise portions of two polarization-maintaining optical fibers extending side by side; and elongating the heated portions to thereby form a fused-elongated section, wherein elongation is terminated when the cyclic changes in a coupling ratio of two polarized waves according to an elongation length at a wavelength in use are both within first two cycles, so that the coupling ratio of one of the polarized waves is equal to or less than 10% and the coupling ratio of the other one of the polarized waves is equal to or greater than 90%.
This invention has the following advantages.
This invention can provide a polarization-maintaining optical fiber coupler whose coupling ratio has a large dependency on polarization with a short elongation length. It is therefore effective to use this polarization-maintaining optical fiber to prepare a PBS.
The short elongation length can permit the polarization-maintaining optical fiber coupler to have a large mechanical strength. Further, it is possible to reduce the number of times the X polarized wave or Y polarized wave is coupled from one polarization-maintaining optical fiber to the other one (the number of transfer times), thus resulting in a low loss.
Furthermore, this invention can provide a polarization-maintaining optical fiber coupler whose coupling ratio has a large polarization dependency over a wide wavelength band. It is therefore possible to provide a PBS effective in preparing an optical circuit which, for example, simultaneously separates light of multiple wavelengths to different polarized waves or couple polarized waves.
It is also possible to provide a polarization-maintaining optical fiber coupler with a smaller excess loss by using polarization-maintaining optical fibers which have a large diameter A.